A Conspicuous Clay Ovoid in Nakhla: Evidence for Subsurface Hydrothermal Alteration on Mars with Implications for Astrobiology

Abstract

A conspicuous biomorphic ovoid structure has been discovered in the Nakhla martian meteorite, made of nanocrystalline iron-rich saponitic clay and amorphous material. The ovoid is indigenous to Nakhla and occurs within a late-formed amorphous mesostasis region of rhyolitic composition that is interstitial to two clinopyroxene grains with Al-rich rims, and contains acicular apatite crystals, olivine, sulfides, Ti-rich magnetite, and a new mineral of the rhoenite group. To infer the origin of the ovoid, a large set of analytical tools was employed, including scanning electron microscopy and backscattered electron imaging, wavelength-dispersive X-ray analysis, X-ray mapping, Raman spectroscopy, time-of-flight secondary ion mass spectrometry analysis, high-resolution transmission electron microscope imaging, and atomic force microscope topographic mapping. The concentric wall of the ovoid surrounds an originally hollow volume and exhibits internal layering of contrasting nanotextures but uniform chemical composition, and likely inherited its overall shape from a preexisting vesicle in the mesostasis glass. A final fibrous layer of Fe-rich phases blankets the interior surfaces of the ovoid wall structure. There is evidence that the parent rock of Nakhla has undergone a shock event from a nearby bolide impact that melted the rims of pyroxene and the interstitial matter and initiated an igneous hydrothermal system of rapidly cooling fluids, which were progressively mixed with fluids from the melted permafrost. Sharp temperature gradients were responsible for the crystallization of Al-rich clinopyroxene rims, rhoenite, acicular apatites, and the quenching of the mesostasis glass and the vesicle. During the formation of the ovoid structure, episodic fluid infiltration events resulted in the precipitation of saponite rinds around the vesicle walls, altered pyrrhotite to marcasite, and then isolated the ovoid wall structure from the rest of the system by depositing a layer of iron oxides/hydroxides. Carbonates, halite, and sulfates were deposited last within interstitial spaces and along fractures. Among three plausible competing hypotheses here, this particular abiotic scenario is considered to be the most reasonable explanation for the formation of the ovoid structure in Nakhla, and although compelling evidence for a biotic origin is lacking, it is evident that the martian subsurface contains niche environments where life could develop.

Opal‐A in the Nakhla meteorite: A tracer of ephemeral liquid water in the Amazonian crust of Mars

“…The petrographic context of the ferric (oxy)hydroxide (i.e., irregular patches and lamellae with siderite) indicates that it was the last of the three components, and formed by oxidation of the carbonate. This interpretation for the origin of the ferric (oxy)hydroxide agrees with previous reports of alteration of the Nakhla carbonates (Saxton et al 2000;Chatzitheodoridis et al 2014), and Treiman and Gooding (1991) likewise concluded that patches and veinlets of hematite and ferrihydrite were the last of the secondary minerals to have formed in Nakhla veins. The presence within ferric (oxy)hydroxide patches of Si, Cl, and K, which are otherwise absent from the siderite, shows that the fluid responsible for alteration of the carbonates had interacted with preexisting Fe-Mg silicate, and so further supports the sequence of events outlined above.…”

“…; Chatzitheodoridis et al. ), and Treiman and Gooding () likewise concluded that patches and veinlets of hematite and ferrihydrite were the last of the secondary minerals to have formed in Nakhla veins. The presence within ferric (oxy)hydroxide patches of Si, Cl, and K, which are otherwise absent from the siderite, shows that the fluid responsible for alteration of the carbonates had interacted with pre‐existing Fe‐Mg silicate, and so further supports the sequence of events outlined above.…”

Thermodynamics, Disequilibrium, Evolution: Far-From-Equilibrium Geological and Chemical Considerations for Origin-Of-Life Research

“…Earth of course still has such conditions; Venus may have harbored such conditions very early on; and there is evidence that Mars at least had surface water in the Noachian. For example, Martian shergottites display signs of aqueous interaction extending to the last few million years, and even now there are vestiges of a water table (Webster et al 2014 ;Cockell 2014 ;Chatzitheodoridis et al 2014 ). Sustained water-rock interaction may also occur on icy moons that host liquid water oceans in contact with a mafic silicate layer, thereby producing chemical disequilibria similar to those likely present on the early Earth (Vance et al 2007;Hsu et al 2015).…”

Oxygen Isotope Thermometry of DaG 476 and SaU 008 Martian Meteorites: Implications for Their Origin

“…therein) of major rock-forming minerals (i.e., px-ol), transformation of plagioclase to maskelynite ( [30] and refs. therein), and formation of melt pockets, veins, and bubbles in the glass [31,32]. Shock features in olivine and pyroxene include planar deformation features, twinning, and mosaicism and these are abundant in both DaG 476 and SaU 005 [33].…”